Redbud (Cercis L. spp., Fabaceae: Caesalpiniodeae: Cercideae), are popular ornamental small trees or shrubs valued commercially for their showy early spring bloom, heart-shaped leaves, and adaptability to diverse environmental conditions. Each year, about one million redbud plants are sold in the United States, with a market value of $27 million (USDA-NASS, 2015). The genus Cercis consists of about 10 species that occur in temperate regions around the world (Chen et al., 2010; Davis et al., 2002; Fritsch et al., 2009; Robertson, 1976). Cercis canadensis (eastern redbud) is a small tree native to the eastern half of the United States and is widely cultivated in the United States, with more than three dozen cultivars available in the trade (Wadl et al., 2012). These cultivars display wide variation in plant architecture, plant size, and flower and leaf colors (Roberts et al., 2015). Despite the popularity of this species, large-scale production of eastern redbud can be difficult due in part to inability of most cultivars to readily root from cuttings, and its susceptibility to canker caused by the fungus Botryosphaeria riibis Grossenbacher and Duggar (Dirr, 1998; Dirr and Heuser, 1987; Pooler et al., 2002; Raulston, 1990).
In China, five species of redbud are distributed across various provinces (Chen et al., 2010); C. chinensis (Chinese redbud) has wide distribution in southern China, and is the most commonly cultivated Asian species in the United States (Raulston, 1990). It is characterized by a rounded, shrubby habit, fast growth rate, and short juvenility period compared with other species in production (Wadl et al., 2012). Cercis chingii, which is found in southeastern China, is also cultivated as an ornamental plant in the United States (Wadl et al., 2012). One clone of C. glabra (syn. C. yunnanensis) has been used in the USNA redbud program, primarily because of its amenability to manipulation in tissue culture (Cheong and Pooler, 2003; Nadler et al., 2012). Cercis racemosa has also been used in the USNA redbud breeding program for its showy racemose flowers, ability to propagate from cuttings, and increased tolerance to canker (Pooler and Dix, 2001; Pooler et al., 2002). In support of the breeding program, the USNA has amassed a diverse collection of Cercis germplasm collected in North America and Asia, as well as representative redbud cultivars sold in the United States. Because of their increasing popularity in cultivation in the United States, we were particularly interested in clarifying the identity and diversity of the Asian Cercis accessions in our collection. Identification and taxonomic placement of various Cercis species based on morphological characters alone can be problematic, as these characters can be quantitative and continuous and influenced by the environment (Wadl et al., 2012). However, SSR markers that are randomly distributed throughout the nuclear genome have the appropriate distribution and frequency to provide valuable information both within and between the species (Egan et al., 2012; Wadl et al., 2012). Genomic SSRs have been developed in C. canadensis and C. chinensis by Wadl et al. (2012) and Gong et al. (2012), respectively. They reported that 55 C. canadensis SSR loci exhibited cross-species transfer; whereas only four to six C. chinensis SSR loci could be used for different species. The objective of this study was to use SSR loci from C. canadensis to clarify the identity and diversity of the USNA’s Asian Cercis accessions.
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